MobiPocket does have
PRCGEN Documentation, which provides some information about the supported HTML.
You have probably already seen
MobiPocket TOC using mobigen and
Images in MobiPocket. In particular, a toc.html appears to be required for mobigen to create a TOC and it is inserted at the end of the .mobi file. An automatic TOC would be a useful addition, and yet another reason to prefer html2mobi over mobigen.
I have never seen the hisrc attribute (
Image support and display) used for an image in an actual MOBI file, but it might be one way to add a larger image to a MOBI file while maintaining backward compatibility. It might be enough, though, to have a default image size, or have html2mobi honor width & height larger than the image by rescaling the image (note that the reader ignores width & height larger than the image).